Investments (Summary of Short-Term Investments) (Detail) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
Jun. 30, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Amortized cost	$ 19,394	$ 46,632
Unrealized Gains	0	0
Unrealized Losses	(23)	(77)
Estimated fair value	$ 19,371	$ 46,555
Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Maturity (or less) (in years)	1 year	1 year
Amortized cost	$ 10,007	$ 32,922
Unrealized Gains	0	0
Unrealized Losses	(7)	(55)
Estimated fair value	$ 10,000	$ 32,867
Certificates of deposit
Debt Securities, Available-for-sale [Line Items]
Maturity (or less) (in years)	1 year	1 year
Amortized cost	$ 3,890	$ 8,216
Unrealized Gains	0	0
Unrealized Losses	0	0
Estimated fair value	$ 3,890	$ 8,216
U.S. Treasury securities
Debt Securities, Available-for-sale [Line Items]
Maturity (or less) (in years)	1 year	1 year
Amortized cost	$ 3,998	$ 3,996
Unrealized Gains	0	0
Unrealized Losses	(14)	(18)
Estimated fair value	$ 3,984	$ 3,978
Debt securities of U.S. government-sponsored agencies
Debt Securities, Available-for-sale [Line Items]
Maturity (or less) (in years)	1 year	1 year
Amortized cost	$ 1,499	$ 1,498
Unrealized Gains	0	0
Unrealized Losses	(2)	(4)
Estimated fair value	$ 1,497	$ 1,494

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Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

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-SubTopic 10
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Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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